Answer to Question #8654 Submitted to "Ask the Experts"

Category: Nuclear Medicine Patient Issues — Therapeutic Nuclear Medicine

The following question was answered by an expert in the appropriate field:

Q
My husband had his thyroid removed and will be undergoing radioactive iodine treatment. My husband is on synthyroid and gets two shots two days prior to therapy.

I will be 14 weeks pregnant at the time my husband goes for treatment. He will be in isolation at the hospital for two days. We also have a four-year-old daughter. What precautions should we take upon his return home? Should he stay away from home for a longer period, considering the circumstances?
A
I assume from what you have told me that your husband is getting recombinant human TSH (thyroid stimulating hormone), which means that he will not be clinically hypothyroid, and will have an eight-hour biological renal clearance half-time (7.6 h effective half-life) for the iodine-131 that does not go to his thyroid.

Two days in the hospital will mean 48/7.6 = 6.3 half-lives, which means that the nonthyroidal dose will be reduced by a factor of 78.7.

Assuming an activity of 3700 MBq (100 millicuries), with about 99 percent being nonthyroidal, means that he will have 74-114 MBq (2-3 mCi) in him at discharge. If he gets 7400 MBq, he will have 148-222 MBq in him.

Let's assume a body burden of 185 MBq. If you sleep separately for a week and are no closer than 1 meter 25 percent of the time, the dose to you (and the fetus) is 0.3 mSv, assuming that there are 74 MBq in the thyroid remnant and that iodine-131 has an effective half-life of eight days.

If you sleep in the same bed when he comes home, assuming a distance of 0.67 meters, and eight hours of sleep a night, that additional dose would be 1 mSv.

So, you could receive about 1.3 mSv if he gets 7400 MBq and you take no precautions at all. If he gets 3700 MBq, you and the fetus only get about 0.65 mSv.

I don't think that any of these doses are an issue and would not worry about them at all. Recall that background is about 0.01 mSv/day, every day of your life. I would caution against contamination from saliva for a few days, which means no sharing cups and the like.

Carol S. Marcus, PhD, MD
Professor of Radiation Oncology and of Radiological Sciences, UCLA
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